Method and device for pre-treatment of substrates

ABSTRACT

Process and apparatus for pre-treatment of a substrate surface in a vacuum by a glow discharge for a subsequent coating process in a vacuum. The process includes maintaining a low pressure glow discharge between the substrate to be pre-treated and a counter-electrode, where the counter-electrode composed of at least a component of the coating to be deposited in the vacuum coating process. The process also includes periodically alternating a polarity of the substrate to act as a cathode or as an anode of the low pressure glow discharge, and individually controlling at least one of pulse length and discharge voltage in both polarities. A frequency of alternation of the polarity is set within a range of between 1 Hz and 1000 kHz. The apparatus includes an evacuatable vacuum chamber, a substrate holder positioned to hold a substrate to be pre-treated, at least one counter-electrode, and an alternating voltage generator coupled to the substrate to be pre-treated and the at least one counter-electrode. The substrate and the counter-electrode are mounted in a potential-free manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Stage of InternationalApplication No. PCT/EP96/05032 filed Nov. 5, 1996, and claims priorityunder 35 U.S.C. § 119 of German Patent Application No. 195 46 826.0,filed Dec. 15, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and a related device for thepre-treatment of electrically conductive or non-conducting substrates ina vacuum. The preferred field of application is the treatment ofsurfaces that require an adhesive coating in a subsequent vacuum coatingprocess. Tools, made of steel, hard metal or ceramic, for example aretreated in accordance with the method.

2. Discussion of Background Information

It is known (R. A. Haefer: Oberflachen- und Dunnschicht-Technologie,Vol. I, pp. 30 ff., Springer-Verlag, Berlin 1987) that substrates to becoated in a vacuum have to be exposed to a multi-step treatment process.It usually consists of one or several mechanical or chemical cleaningsteps, as well as a subsequent vacuum treatment in the coating facility.Here the interfering surface layers, for example water films or thinoxide layers, are removed, if needed, the nucleation locations for thelayer to be deposited are created, or even adhesion-promotinginter-coatings are applied.

Different methods of vacuum pre-treatment have been developed, dependingon the electrical conductance of the substrates. Electricallynon-conducting substrates, such as glass or oxide ceramics, are oftencleaned by a glow discharge, which is ignited between the groundedsubstrate fastener as the anode and a special glow electrode as thecathode in a working gas, preferably Argon, with a pressure around 10Pa. The substrate is thereby hit by electrons, which primarily cause thedesorption of contaminant layers, in particular of the water film, andcause the substrate to heat up. Condensation nuclei can furthermore becreated by the sputtering of the electrode material. The primarydisadvantage of this method is based on the low current densities thatcan be achieved, and thereby its low efficiency. It is furthermoreimpossible to remove thermally stabile contamination layers, for examplemade of oxides, with the aid of electron bombardment of the substrate.

Electrically conductive substrates are pre-treated in the simplestmanner by using an sputtering process. The discharge required for thisis ignited between the negatively biased substrate as the cathode andthe grounded vacuum chamber, or an etching electrode, as the anode. Theforeign atoms are removed by ion bombardment of the substrate and thesurface of the substrate is activated. Additional particles andradiation quanta of the plasma assist the activation by interacting withthe substrate surface.

It is known that the effect of the pre-treatment can be increased,depending on the substrate material, by using reducing or oxidizing gasmixtures (DE 31 44 192).

It is furthermore known that the overlay of a magnetic field, inparticular in accordance with the magnetron principle, leads to thestrengthening of the discharge and thereby to an intensification of thepre-treatment (DD 136 047).

The substrate pre-treatment with a direct current-glow discharge oftentimes exhibits process instabilities. They are caused by thenon-conductive areas of the substrate surface, for example isolatedareas of oxide, to electrically charge up, and by the sudden release ofthe glow discharge in a resulting arc discharge. This phenomenon, alsoreferred to as "arcing" does not only impede the removal of thecontaminant layers, but also leads to the occurrence of localizeddamages of the substrate surface.

A known method, which overcomes these disadvantages, is thepre-treatment in a RF-plasma, preferably at a frequency of 13.56 MHz.For non-conductive substrates, a RF-plasma is generally required. Withthe proper design of the electrode surfaces, a so-calledself-bias-voltage occurs, which creates a beam of accelerated ions tothe substrate. Numerous elementary processes are thereby triggered,leading to the cleaning of the substrate surface by sputtering and to anactivation of the surface. All RF-processes have the disadvantage ofexhibiting low efficiency, high energy losses during the electricaladaptation, and an elaborate technical outlay. In particular forsubstrates with large surfaces, a homogenous substrate pretreatment ispractically impossible.

It is also known that a pre-treatment of substrates in vacuum can beachieved by using charged-particle sources. With the aid of electronbeam sources, a heating of the substrate and a desorption of volatileadsorption layers is essentially achieved. By bombarding the substrateswith ion sources or plasma sources, non-volatile surface layers can alsobe removed by sputtering (DD 292 028; DE 37 08 717 C2).

The bombardment of the substrate with metal ions (Kadlec et al., Surf.Coat. Technol., 54/55, 1992, 287-296), still results in the creation ofa coating mixture of substrate atoms and implanted foreign atoms. Theprocess is therefore also known as "ion mixing". It leads to very goodadhesion of the coatings applied subsequently in the vacuum.

All of the stated beam processes for the pre-treatment of substrateshave the disadvantage of requiring very elaborate equipment, and aretherefore associated with high costs. This is especially true forsubstrates with large surfaces.

For special applications, for example the pre-treatment of polymersurfaces, the use of microwave-plasmas is known. These do not achieve aremoval of contamination layers, but rather the modification of thephysical and chemical adhesion properties between the substrate and thecoating material. The effectiveness is extremely dependent on thematerial and tied to the chemical nature of the substrate and thecoating material.

SUMMARY OF THE INVENTION

The task of the invention is to propose a method and a related devicefor the pre-treatment of electrically conducting and non-conductingsubstrates, which significantly reduces the technical outlay for thesubstrate pre-treatment, relative to a treatment in RF-plasma or withthe aid of beam sources. The effectiveness of the pre-treatment isfurthermore to be improved and to be adjusted to the properties of thesubstrate material, as well as the coating to be applied subsequently.The process should be able to be combined with the actual vacuum coatingprocess without causing any problems.

The present invention provides a process for the pre-treatment of asubstrate surface in a vacuum by a glow discharge for a subsequentcoating process in a vacuum. A low pressure glow discharge is maintainedbetween the substrate to be cleaned and the counter-electrode, which ismade, e.g., of a material of or at least a component of the coating tobe deposited in the vacuum coating process. The substrate periodicallyacts as a cathode or as an anode of the low pressure glow discharge inan alternating manner, and the frequency of the alteration of polarityis set in the range of between 1 Hz to 1000 kHz, and preferably between20 and 50 Hz. Pulse lengths and/or discharge voltage can be individuallycontrolled in both polarities.

The present invention also provides a device for performing the processof the invention. The device includes a vacuum chamber that can beevacuated, in which the substrate to be pre-treated is fastened and inwhich at least one counter-electrode is arranged, and a power supply.The substrate and the counter-electrode are mounted potential-free andconnected with an alternating voltage generator.

The method in accordance with the invention causes the substrate to behit with a beam of accelerated electrons and ions in an alternatingfashion. The substrate is furthermore exposed permanently to theinteractions between additional particles and radiation quanta of aplasma, in particular with high-energy neutral particles and radiationfrom the plasma. The chemical and physical effect of a portion of theions making impact is determined by the selection of the electrodematerial. The counter-electrode is preferably made of the material ofthe coating to be applied subsequently to the material or at least of acomponent of the coating material. A reducing gas, for example hydrogen,can be fed into the vacuum chamber during the pre-treatment process,becoming activated or ionized in the plasma, supporting the pretreatmentprocess through chemical reactions. Overall, a very complex physical andchemical reaction on the substrate surface and in the regions near thesurface is caused by this method.

Characteristic for the process is its suitability for electricallyconducting and non-conducting substrates. The frequency of the changingpolarity of the power supply prevents the non-conducting regions of thesubstrate surface to charge up. The "arcing", present in direct currentplasma, is thereby avoided. The frequency of the changing polarity isfurthermore determined, such that during each pulse, the ions of theplasma receive a sufficiently high amount of energy from the electricalfield of the discharge. It is particularly advantageous to be able todetermine the type and intensity of the elementary processes, preferablyoccurring on the substrate by adjusting the electrical parameters of thepower supply device. The chosen relationship of the pulse lengths ofboth polarities determine the relationship of electron- and ion beam tothe substrate. By choosing the discharge voltage in both polarities, theenergy distribution of the electrons and ions is determined. One therebyactively influences to what degree such elementary processes, such asdesorption of loosely bound adsorbates, sputtering of contaminantlayers, sputtering of the substrate material, heating of the substrateand thereby an elevation of the lateral mobility, diffusion processes,and implantation of electrode material in the substrate region close tothe surface, are executed. The pre-treatment process is thereby not onlya physical and/or chemical cleaning process of the substrate, but isalso able to predetermine the adhesion and structure of the layer to beapplied in a specific manner.

It can also be advantageous to operate the stated glow-discharge in anoxidizing gas mixture. This is particularly true when the substrate andlayer material, due to their chemical nature, undergo oxidative bonding.

The process can be operated particularly advantageously when thecounter-electrode is arranged in a field of a special type of magnetassembly--a known magnetron arrangement. The plasma density and theefficiency of the pre-treatment can thereby be increased by orders ofmagnitude.

In cases in which the subsequent deposition of a coating is to occur bymagnetron-sputtering, it is particularly advantageous to use one of thesputtering sources designed for the layer deposition ascounter-electrode for the pre-treatment of the substrates.

The device for executing the process in accordance with the inventionessentially consists of a vacuum chamber, in which the substrate to betreated is fastened, the counter-electrode, an evacuating unit, and apower supply. An alternating voltage generator, whose terminals areconnected to the substrate and to at least one counter-electrode, servesas the power supply.

In addition, the device consists of means by which a working gas can belet into the process chamber and means by which the pressure can beregulated, as they are generally known from plasma processes.

It is effective to construct the alternating voltage generator as abipolar-pulse-generator, whose pulse lengths and/or output voltages canbe adjusted individually for both polarities. In a simplified way, thealternating voltage generator can also be constructed by hooking up asine wave generator with an adjustable DC voltage source. By overlayingboth voltages, it is possible to adjust the ratio of the electron- andion current on the substrate for the process.

For an efficient device for executing the process, it is functional toprovide a water-cooled counter-electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The object of the invention is explained in more detail with twoembodiments. The corresponding drawings show:

FIG. 1: a device for the pre-treatment of an electrically non-conductingsubstrate with a bipolar-pulse-generator,

FIG. 2: an arrangement for the pre-treatment of an electricallyconducting, partially oxidized substrate with a sine wave generator.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In FIG. 1, a non-conductive substrate 2, made of an oxide ceramic, isplaced in a fastener 3 of a process chamber 1. A known vacuum pumpsystem is connected to the evacuation line 4 of the process chamber. Anargon-oxygen gas mixture with an oxygen content of 10 vol % is let intothe process chamber 1 with the aid of a regulator valve 5, and regulatedto a pressure of 0.5 Pa. A counter-electrode 6 is arranged parallel tothe substrate 2 at a distance of 100 mm. It is located in the magneticfield of a magnet coil 7. The counter-electrode 6 is made of specialsteel; the coating to be deposited onto the substrate 2 shall also madeof special steel.

For the pre-treatment of the substrate surface, fastener 3, andtherefore also the substrate 2, and the counter-electrode 6 areconnected in a potential-free manner and connected to the terminals ofan alternating voltage generator 8 that is a bipolar-pulse-generator.Its maximum output measures 10 kW with a maximum output voltage of 1500V for each polarity. The alternating voltage generator 8 is operatedwith a set frequency for alteration of polarity of 50 kHz. The pulselengths for the cathodic and the anodic polarity of the substrate 2 areadjusted at a ratio of 20:1. The output voltage for both polarities isset for 1200 V.

With a pre-treatment time of 2 minutes, an average removal of 20 nm onthe substrate 2 and a temperature elevation of 410° C. is achieved. Inthe subsequent coating process, a special steel layer of 20 μmthickness, exhibiting an extremely high degree of adhesion and density,is deposited on the substrate 2 with the aid of electron beamevaporation. A cross-section of the coated substrate 2 shows that an"intermixing zone" of approximately 200 nm was formed as a result,which, aside from the substrate material, also contains metalliccomponents. It was obviously created by implantation and diffusion.

In FIG. 2 the metallic conducting substrates 2 are to be coated with atitanium-oxide layer of 10 μm using reactive magnetron sputtering. Thesubstrates 2, made of cemented carbide, are fastened in a processchamber 1 on a rotating substrate carrier. A known vacuum pump system isconnected to the process chamber 1 via the evacuation line 4. Theregulator valve 5 is designed to let in the argon gas mixture with ahydrogen content of 5 vol %. The total pressure is held constant at avalue of 1 Pa. Two opposite magnetron-sputtering sources 12, withtitanium targets 13, are arranged at an average distance of 80 mm to thesubstrate carrier 11. They also serve the subsequent coating process ofthe titanium-oxide layer by magentron-sputtering. To execute thesubstrate pre-treatment, an alternating voltage generator, such as acombination of a sine wave generator 14 and an adjustable DC voltagesource 15. is switched between the substrates 2 and themagnetron-sputtering sources 12, which are connected to parallelcircuitry, in a potential-free manner. The targets of themagnetron-sputtering sources 12 therefore act as the counter-electrodefor the pre-treatment process.

The sine wave generator 14 is operated at a frequency of 50 kHz and anoutput voltage of 1500 V. The DC voltage source 15 is set at a value of650 V. Overlaying the two voltages results in the pulse lengths as wellas the discharge voltages to be as high or significantly higher with acathodic polarity than with an anodic polarity. Alternatively, the pulselengths can be higher in the anodic polarity than in the cathodicpolarity. An average removal of less than 10 nm on the substrate 2 and atemperature increase around 300° C. is achieved with a pre-treatmenttime of 10 minutes. The atomized coatings, deposited after thistreatment, exhibit very high adhesion. The scratch test did not revealany layer separation under 30 N, the maximum load of the equipment.

What is claimed is:
 1. A process for pre-treatment of a substratesurface in a vacuum by a glow discharge for a subsequent coating processin a vacuum, the process comprising:maintaining a low pressure glowdischarge between the substrate to be pre-treated and acounter-electrode, the counter-electrode composed of at least acomponent of the coating to be deposited in the vacuum coating process;periodically alternating a polarity of the substrate to act as a cathodeor as an anode of the low pressure glow discharge; and individuallycontrolling at least one of pulse length and discharge voltage in bothpolarities, wherein a frequency of alternation of the polarity is setwithin a range of between 1 Hz and 1000 kHz.
 2. The process inaccordance with claim 1, wherein the frequency of alternation is between20 and 50 kHz.
 3. The process in accordance with claim 1, furthercomprising:magnetically enhancing the low pressure glow discharge inaccordance with the magnetron principle.
 4. The process in accordancewith claim 1, further comprising:maintaining the low pressure glowdischarge in an inert gas.
 5. The process in accordance with claim 4,wherein the inert gas is argon.
 6. The process in accordance with claim1, further comprising:maintaining the low pressure glow discharge in areducing gas mixture including hydrogen.
 7. The process in accordancewith claim 1, further comprising:maintaining the low pressure glowdischarge in an oxidizing gas mixture including oxygen.
 8. The processin accordance with claims 1, further comprising:setting an anodicpolarity pulse length of the substrate higher than a cathodic polaritypulse length of the substrate.
 9. The process in accordance with claims1, further comprising:setting a cathodic polarity pulse length of thesubstrate as high or higher than an anodic polarity pulse length of thesubstrate.
 10. An apparatus for pre-treating a substrate in a vacuum bya glow discharge for a subsequent coating process in a vacuum, theapparatus comprising:an evacuatable vacuum chamber; an electrodecomposed of the substrate to be pre-treated; at least onecounter-electrode; and an alternating voltage generator coupled to thesubstrate to be pre-treated and the at least one counter-electrode,wherein the substrate to be pre-treated and the counter-electrode aremounted within the vacuum chamber.
 11. The apparatus in accordance withclaim 10, wherein the substrate to be pre-treated is adapted such that apolarity of the substrate to be pre-treated periodically alternates toact as a cathode or as an anode of the low pressure glow discharge. 12.The apparatus in accordance with claim 10, wherein the alternatingvoltage generator comprises a bipolar pulse generator with at least oneof an individually adjustable pulse length and individually adjustableoutput voltages for both polarities.
 13. The apparatus in accordancewith claim 10, wherein the alternating voltage generator comprises asine wave generator coupled to a controllable DC voltage source.
 14. Theapparatus in accordance with claim 10, further comprising at least onemagnetron-sputter source for use in the vacuum coating process followingthe pre-treatment process, wherein the magnetron-sputter source isswitchable as the counter-electrode.
 15. The apparatus in accordancewith claim 10, wherein the counter-electrode is water-cooled.